Cable TV systems are becoming increasingly commonplace in urban areas where they provide a medium for the delivery of video services to customers. These systems have a large amount of spare bandwidth which the system providers can offer for the transport of additional services such as telephone traffic. A recent development of this telephony application has been a proposal to carry cellular communications traffic, e.g. GSM or PCS traffic, over the cable network. In such an arrangement, a cellular base station is coupled via the cable network to a number of antennas to provide communication with mobile units in the service area of the antennas. A proposed system for cellular communication over a CATV network is described by R W Donaldson in IEEE Transactions on Vehicular Technology, vol. 43 no. 3, pp666-70.
The potential use of CATV systems to carry cellular communications traffic has been restrained by a number of technical problems. Firstly, there is the problem of the limited dynamic range of most cable systems which results in significant performance degradation when handling cellular communications traffic. A typical cellular system requires a nominal dynamic range of approximately 80 dB, whereas a typical cable system offers a dynamic range of about 30 dB for the downstream path or down link, and between 20 and 45 dB on the upstream path or uplink. The 30 dB restriction on the down link is acceptable as cellular systems such as the GSM system can accommodate a signal to noise ratio as low as 9 dB. However, since the level at which uplink signals are directed into the cable sysytem is dependent on the distance between a mobile terminal and its associated antenna, any dynamic range restriction on the uplink results in a severe mismatch with the minimum sensitivity level of the cellular receiver. Under conditions that can be reasonably expected in a CATV system, this uplink dynamic range restriction reduces the effective rtansmission distance to an unacceptably low value. Secondly, cable systems intended for the transmission of mobile commmunications traffic are subject to the so-called near-far problem. Within a mobile system it is not possible to guarantee the signal level received at an antenna from a mobile. Consequently it is possible that low level transmissions from a wanted distant transmitter will be swamped by unwanted high power signals from a local transmitter operating on an adjacent channel. In a cable system, this problem is exacerbated by the inherent dynamic range limitation.
The object of the invention is to minimise or to overcome these disadvantages.
It is a further object of the invention to provide an improved communications sysaytem in which cellular communications traffic between mobile terminals and a base station is carried on a CATV network.